one thing to remember is that antenna "gain" is something of a misnomer.
What a directional antenna does is take the RF you would have otherwise
sprayed in all directions and concentrate it into a narrow beam. Inside that
beam your signal strength is the same as an isotropic radiator being fed by
much higher power. That power ratio is your antenna gain in dBi. If the
signal strength comparison was against a half-wave dipole the gain would be
in dBd, which has a couple of dBi gain already since the pattern of a dipole
is a big donut around the wire, with nulls off the ends.

QRP is the art of communicating with a low-power radio. Winning contests is
secondary...

When people play with microwaves it's not uncommon to feed 10 mW to a 50 dBi
gain antenna for an ERP of 100 Watts. But since microwaves tend to be a
field of low power (priced any TWTs lately? :-), high antenna gain and high
ERP, QRP isn't a term people use once the frequency gets up into VHF and
above.